Is Iron Unique in Promoting Electrical Conductivity in MOFs? PDF

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Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Is iron unique in promoting electrical conductivity This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Cite this: Chem. Sci., 2017, 8, 4450 in MOFs?† Open Access Article. Published on 20 April 2017. Downloaded on 6/26/2024 7:08:57 AM. Lei Sun, a Christopher H. Hendon, a Sarah S. Park,a Yuri Tulchinsky,a Ruomeng Wan,a Fang Wang,a Aron Walsh bc and Mircea Dincă *a Identifying the metal ions that optimize charge transport and charge density in metal–organic frameworks is critical for systematic improvements in the electrical conductivity in these materials. In this work, we measure the electrical conductivity and activation energy for twenty different MOFs pertaining to four distinct structural families: M2(DOBDC)(DMF)2 (M ¼ Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+); H4DOBDC ¼ 2,5-dihydroxybenzene-1,4-dicarboxylic acid; DMF ¼ N,N-dimethylformamide), M2(DSBDC)(DMF)2 (M ¼ Mn2+, Fe2+; H4DSBDC ¼ 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid), M2Cl2(BTDD)(DMF)2 (M ¼ Mn2+, Fe2+, Co2+, Ni2+; H2BTDD ¼ bis(1H-1,2,3-triazolo[4,5-b],[40 ,50 -i]dibenzo[1,4]dioxin), and M(1,2,3- triazolate)2 (M ¼ Mg2+, Mn2+, Fe2+, Co2+, Cu2+, Zn2+, Cd2+). This comprehensive study allows us to single-out iron as the metal ion that leads to the best electrical properties. The iron-based MOFs exhibit at least five orders of magnitude higher electrical conductivity and significantly smaller charge activation energies across all different MOF families studied here and stand out materials made from all other metal Received 13th February 2017 Accepted 18th April 2017 ions considered here. We attribute the unique electrical properties of iron-based MOFs to the high- energy valence electrons of Fe2+ and the Fe3+/2+ mixed valency. These results reveal that incorporating DOI: 10.1039/c7sc00647k Fe2+ in the charge transport pathways of MOFs and introducing mixed valency are valuable strategies for rsc.li/chemical-science improving electrical conductivity in this important class of porous materials. metal ions on either the band structure of the underlying Introduction material or the charge density. Metal–organic frameworks (MOFs) that exhibit both high In our previous work we have shown that in two isostructural surface area and electrical conductivity are emerging as a new MOFs made from Mn and Fe, the latter leads to considerably class of materials whose applications reach beyond those improved electrical conductivity by up to six orders of magni- typical of porous solids.1 Reports of electrically conductive tude.5 Additionally, the Fe analogs of M(1,2,3-triazolate)2 (M ¼ MOFs in the last few years have addressed both the funda- Mg2+, Mn2+, Fe2+, Co2+, Cu2+, Zn2+, Cd2+)14,15 and M(TCNQ) (4,40 - mentals: the nature of the charge carriers and the mechanism bpy) (M ¼ Mn2+, Fe2+, Co2+, Zn2+, Cd2+; TCNQ ¼ 7,7,8,8-tetra- of transport,2–6 and the applications: supercapacitors,7 electro- cyanoquinodimethane; 4,40 -bpy ¼ 4,40 -bipyridyl)16 were re- catalysis,8,9 chemiresistive sensing,10,11 and thermoelectrics12 ported as being electrically conductive, although the electrical among others. Certain design principles have emerged from conductivity in the other analogs was not reported. These iso- these studies, focused for instance on targeting either band-like lated reports led us to believe that Fe may play an important and or hopping conductors,13 yet some of the most basic questions unique role in promoting electrical conductivity in MOFs. Here, governing electrical conduction in MOFs are still poorly we compare four structurally distinct classes of MOFs, totalling understood. Most obvious among these is the inuence of the twenty different materials made from eight different metal ions (M ¼ Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+) and show a Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, that Fe does indeed enable high electrical conductivity in Fe- 02139, USA. E-mail: [email protected] containing frameworks. b Department of Materials, Imperial College London, London SW7 2AZ, UK To ascertain the inuence of the metal cation on electrical c Department of Materials Science and Engineering, Yonsei University, Seoul 03722, conductivity systematically, we targeted MOFs that feature South Korea a broad array of chemical connectivity and composition. Four † Electronic supplementary information (ESI) available: Experimental details, families of materials that provide this breadth are M2- PXRD patterns, IR spectra, table of electrical conductivity, table of activation energies, current density vs. electrical eld strength curves, current–voltage (DOBDC)(DMF)2 (M ¼ Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, curves at various temperatures, temperature dependence of electrical Zn2+),17–24 M2(DSBDC)(DMF)2 (M ¼ Mn2+, Fe2+),4,5 M2Cl2- conductivity, 57Fe Mössbauer spectra, magnetic susceptibility plots, BET surface (BTDD)(DMF)2 (M ¼ Mn2+, Fe2+, Co2+, Ni2+),25 and M(1,2,3- area analysis, table of various properties of divalent metal ions, and calculation triazolate)2 (M ¼ Mg2+, Mn2+, Fe2+, Co2+, Cu2+, Zn2+, Cd2+).14,15 details. See DOI: 10.1039/c7sc00647k 4450 | Chem. Sci., 2017, 8, 4450–4457 This journal is © The Royal Society of Chemistry 2017  View Article Online Edge Article Chemical Science This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 20 April 2017. Downloaded on 6/26/2024 7:08:57 AM. Fig. 1 Portions of crystal structures of four families of MOFs emphasizing pores (top) and coordination environment of metal ions (bottom). H atoms and part of DMF molecules have been omitted for clarity. The structure of Mn2(DSBDC)(DMF)2 is shown in Fig. S2 in the ESI.† The rst three families of MOFs display honeycomb structures bound DMF completes the octahedral coordination environ- with 1D tubular pores, whereas the M(1,2,3-triazolate)2 mate- ment of the metal ions in these materials (Fig. S4†). rials exhibit cubic structures with three-dimensional pore Because some of the MOF crystallites were too small for networks.‡ The metal ions in all these MOFs are formally single crystal studies, electrical properties were measured on divalent and octahedrally coordinated (Fig. 1 and S2†). pressed pellets in all cases using the standard two-contact probe method26,27 at 300 K, under a N2 atmosphere, and in the dark. PXRD analysis of the pressed pellets revealed patterns that Experimental results match those of the original materials (Fig. S5†). Plots of the All Mn2+-, Fe2+-, and Co2+-based materials were synthesized observed current density (J) versus electric eld strength (E) for under air-free conditions. Literature procedures were available all MOFs are shown in Fig. S6,† and the electrical conductivity for all materials studied here, with the exception of Fe2Cl2- values are summarized in Fig. 2 and Table S1.† The Fe-based (BTDD)(DMF)2 (MIT-20-Fe), which was synthesized by adapting MOFs exhibit electrical conductivity on the order of 108–106 a strategy similar to the preparation of the Mn, Co, and Ni S cm1, whereas the observed electrical conductivity in all other analogs.25 Its structure was assigned on the basis of powder X- MOFs is six orders of magnitude lower, on the order of 1014– ray diffraction (PXRD) analysis, which revealed a pattern that 1012 S cm1. matches those of the other analogs (Fig. S3c†). To ensure To understand the inuence of Fe on the electronic struc- consistency, all MOFs were soaked successively in dry and tures of these MOFs, we measured the activation energy (Ea) for degassed DMF and dichloromethane (DCM) under air-free each material by collecting current–voltage (I–V) curves between conditions, and evacuated at 100  C under vacuum for 2 h. 300 K and 350 K under vacuum and in the dark (Fig. S7–S26†). The evacuated materials were kept in a N2-lled glovebox. PXRD Plotting the electrical conductivity versus temperature for each and elemental analyses conrmed that all materials retain their MOF indicated thermally activated electrical conduction in all structural and compositional integrity as well as phase purity cases (Fig. S27†).28 The activation energies were extracted by during these manipulations (Fig. S3†). As reported previously, tting the electrical conductivity–temperature relationships to Fe2(DSBDC)(DMF)2 undergoes a spontaneous structural the Arrhenius law (see ESI†), and are summarized in Fig. 3 and distortion (i.e. a “breathing” deformation) but maintains its Table S2.† Here again, we found that the Fe analogs exhibit connectivity.5 Infrared (IR) spectroscopy revealed vibrational signicantly smaller activation energies than the MOFs based modes at approximately 1650 cm1 for M2(DOBDC)(DMF)2, on the other metal ions. M2(DSBDC)(DMF)2, and M2Cl2(BTDD)(DMF)2, conrming that This journal is © The Royal Society of Chemistry 2017 Chem. Sci., 2017, 8, 4450–4457 | 4451  View Article Online Chemical Science Edge Article This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 20 April 2017. Downloaded on 6/26/2024 7:08:57 AM. Fig. 2 Electrical conductivity in M2(DOBDC)(DMF)2, M2- (DSBDC)(DMF)2, M2Cl2(BTDD)(DMF)2, and M(1,2,3-triazolate)2 measured at 300 K, in N2 atmosphere, and in the dark. Surmising that the oxidation and spin state of the Fe centers Fig. 4 57 Fe Mössbauer spectra of Fe2(DOBDC)(DMF)2, Fe2- could affect electrical conductivity, we investigated all Fe-based (DSBDC)(DMF)2, and Fe2Cl2(BTDD)(DMF)2 at 80 K as well as Fe(1,2,3- MOFs by 57Fe Mössbauer spectroscopy. At 80 K, the 57Fe triazolate)2 at 80 and 298 K. All samples were kept in N2 atmosphere. Mössbauer spectra of Fe2(DOBDC)(DMF)2, Fe2(DSBDC)(DMF)2, Black dots represent experimental data, and red curves represent Lorentzian fitting curves. and Fe2Cl2(BTDD)(DMF)2 (Fig. 4) display doublets with isomer shis d ¼ 1.318, 1.172, and 1.099 mm s1, and quadrupole splittings |DEQ| ¼ 2.749, 3.218, and 1.923 mm s1, respectively. These isomer shis can be unambiguously assigned to high- spin (S ¼ 2) Fe2+ centers.29 At 80 K, the 57Fe Mössbauer spec- trum of Fe(1,2,3-triazolate)2 exhibits a singlet with d ¼ 0.384 mm s1 and no quadrupole splitting. The singlet feature, characteristic of high symmetry (Oh) Fe centers, persists at 298 K although d decreases slightly to 0.336 mm s1 (Fig. 4). Isomer shi values in the range 0.3–0.4 mm s1 can be assigned to either Fe3+ or low-spin (S ¼ 0) Fe2+.29 We assign this singlet to low-spin (S ¼ 0) Fe2+ because elemental analysis for Fe(1,2,3- triazolate)2 agrees with a majority of Fe2+. However, we cannot rule out the presence of Fe3+ that are not detectable by 57Fe Mössbauer spectroscopy (under our conditions, we estimate the sensitivity at approximately 1%). To further probe the possible existence of Fe3+, we performed electron paramagnetic resonance (EPR) experiments, which are sensitive to ppm-level concentrations of Fe3+ under our conditions. The EPR spectrum of Fe(1,2,3-triazolate)2 displayed a broad signal at g z 2.0 and a sharp signal at g z 4.3 (Fig. 5). These are diag- nostic of high-spin (S ¼ 5/2) Fe3+ centers.30,31 Although EPR spectra of Fe2(DOBDC)(DMF)2, Fe2(DSBDC)(DMF)2, and Fe2Cl2(BTDD)(DMF)2 revealed only very broad signals, likely due to Fig. 3 Activation energies of M2(DOBDC)(DMF)2, M2(DSBDC)(DMF)2, signicant spin–spin relaxation stemming from closely connected M2Cl2(BTDD)(DMF)2, and M(1,2,3-triazolate)2 measured at 300–350 K, in vacuum, and in the dark. high-spin Fe2+ ions, these materials are even more air-sensitive 4452 | Chem. Sci., 2017, 8, 4450–4457 This journal is © The Royal Society of Chemistry 2017  View Article Online Edge Article Chemical Science and Fe(1,2,3-triazolate)2 were 248, 83, 365, and 443 m2 g1, respectively (Fig. S31, Table S3†), in line with previous reports and the values expected for each structural type.5 Electronic structure calculations To further probe the inuence of Fe on the electrical properties of MOFs, we evaluated the electronic structures of the M2(DOBDC), M2(DSBDC), and M(1,2,3-triazolate)2 families This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. using density functional theory (DFT) calculations.§ The unit cell of the M2Cl2(BTDD) family proved too large and we were Open Access Article. Published on 20 April 2017. Downloaded on 6/26/2024 7:08:57 AM. unable to compute its properties with reasonable computa- tional resources. Owing to the structural similarities between the innite Fe-based chains in Fe2(DEBDC) (E ¼ O, S) and Fe2Cl2(BTDD) we infer that computational results from the former may be extended to understand the latter. In most cases, our studies yielded intuitive electron energies as presented in Fig. 5EPR spectrum of Fe(1,2,3-triazolate)2 collected at 77 K and in N2 Fig. 7.{ One intriguing exception was found for the electronic atmosphere. structure of Co2(DOBDC): previous reports computed with the PBEsol functional showed a ground state high-spin (S ¼ 3/2) electronic structure. In our hands, PBEsol indeed converges to than Fe(1,2,3-triazolate)2.22 It is therefore reasonable to operate a high-spin structure, but higher level computational analysis under the assumption that all of our Fe MOFs contain Fe3+. with the HSE06 functional surprisingly revealed the contrary: Indeed, 57Fe Mössbauer spectroscopic studies revealed that expo- a high-spin Co2+ structure did not converge, and a stable sure of Fe2(DSBDC)(DMF)2 and Fe2Cl2(BTDD)(DMF)2 to air minimum was found only for the low-spin (S ¼ 1/2) congura- immediately generates a large amount of Fe3+ (>70%, Fig. S28 and tion. This could be due to the systematic differences in equa- S29†), whereas exposing Fe(1,2,3-triazolate)2 to air for at least one tions of state that arise from the use of different functionals.32 month did not change the isomer shi signicantly (d ¼ 0.340 mm We could not probe this hypothesis given the extremely s1) (Fig. S30†). expensive calculation required to geometrically optimize the N2 sorption measurements for the Fe-based materials Co2+-containing MOF with a hybrid functional. revealed Type I isotherms for microporous Fe2(DOBDC)(DMF)2 A summary of the band alignments and accompanying pro- and Fe(1,2,3-triazolate)2, and a Type IV isotherm for meso- jected density of states (PDOS) of the computed MOFs are pre- porous Fe2Cl2(BTDD)(DMF)2, with comparatively little gas sented in Fig. 7. The band structures for the M(1,2,3-triazolate)2 uptake for Fe2(DSBDC)(DMF)2 (Fig. 6). The corresponding Bru- materials are superimposed over the schematic band alignment nauer–Emmet–Teller (BET) apparent surface areas for Fe2- diagrams, to depict the electronic bandwidth. The valence band (DOBDC)(DMF)2, Fe2(DSBDC)(DMF)2, Fe2Cl2(BTDD)(DMF)2, (VB) maximum energy (EVBM), conduction band (CB) minimum energy (ECBM), and band gap (Eg) of each MOF are listed in Table S4.† The energy levels were referenced to an internal vacuum level using a method reported previously.33 In the M2(DOBDC) family, closed-shell ions, Mg2+ and Zn2+, contribute little to either VB or CB (Fig. 7a). In contrast, open- shell ions, Mn2+, Fe2+, Ni2+, and Cu2+, participate in both VB and CB. More electronegative metal ions, such as Cu2+, contribute to a greater extent to the CB and also lower ECBM, whereas more electropositive metals have greater contribution to the VB and raise EVBM. For instance, Fe-based orbitals dominate the VB of Fe2(DOBDC), which also exhibits the highest EVBM (5.2 eV) and the smallest band gap (Eg ¼ 2.0 eV) in this family. Cu-based orbitals dominate the CB of Cu2- (DOBDC), which exhibits the lowest ECBM (3.9 eV) and the second smallest band gap (Eg ¼ 2.2 eV). All other MOFs in this family exhibit Eg of approximately 3 eV. These results are qualitatively consistent with the experimental observation that the activation energy of Fe2(DOBDC) is smaller than those of other analogues. Fig. 6 N2 adsorption isotherms (77 K) of Fe2(DOBDC)(DMF)2, The trends observed for M2(DOBDC) are reproduced in the Fe2(DSBDC)(DMF)2, Fe2Cl2(BTDD)(DMF)2, and Fe(1,2,3-triazolate)2. M2(DSBDC) family. In Fe2(DSBDC) EVBM is increased by 0.5 eV This journal is © The Royal Society of Chemistry 2017 Chem. Sci., 2017, 8, 4450–4457 | 4453  View Article Online Chemical Science Edge Article This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 20 April 2017. Downloaded on 6/26/2024 7:08:57 AM. Fig. 7 Calculated energy bands and projected density of states (PDOS) of (a) M2(DOBDC), (b) M2(DSBDC), and (c) M(1,2,3-triazolate)2. The electron energies are referenced to the vacuum level using the method presented in ref. 33. EVBM are shown on the top and band gaps are shown in the middle of each sub-figure. and ECBM is decreased by 0.5 eV relative to the Mn analog, triazolate)2 exhibits the lowest ECBM (3.0 eV) and the smallest together giving rise to 1.0 eV difference between the Eg values of band gap (Eg ¼ 2.3 eV). These trends qualitatively agree with the the two materials (Fig. 7b). This is in line with the smaller activation energies determined experimentally: the Mg2+, Zn2+, activation energy observed experimentally for the Fe analog. and Cd2+ materials exhibit similar activation energies that are In the M(1,2,3-triazolate)2 family, closed-shell ions again give generally higher than those of the open-shell systems. bands of different parentage than the open-shell ions. Thus, At rst glance, Fe(1,2,3-triazolate)2 appears to be anomalous Mg2+, Zn2+, and Cd2+ do not participate in the VB or CB, which in this family because its computed Eg is large, which should are primarily ligand-based and give rise to similar band gaps for give rise to high Ea and low intrinsic electrical conductivity, in the respective MOFs (Eg ¼ 5.5–5.9 eV) (Fig. 7c). On the other direct contrast with its experimentally determined low Ea and hand, the PDOS for the Mn2+, Co2+, and Cu2+ analogs show that high electrical conductivity. The computational result appears metal-based orbitals dominate both VB and CB, with negligible to be particularly unusual given that the Fe2+ centers in this contribution from ligand-based orbitals. Charge carriers in material are low-spin (S ¼ 0), and are therefore unlikely to these materials must therefore be localized on the metal ions. contribute high-energy charge carriers. Fe3+ ions, however, As in M2(DOBDC) and M2(DSBDC), EVBM and ECBM are deter- could provide such charge carriers. mined by the electronegativity of the metal ions: Mn(1,2,3- Insight into the effect of Fe3+ on the electronic structure of triazolate)2 exhibits the highest EVBM (4.6 eV), and Cu(1,2,3- Fe(1,2,3-triazolate)2 came from DFT analysis of a hypothetical 4454 | Chem. Sci., 2017, 8, 4450–4457 This journal is © The Royal Society of Chemistry 2017  View Article Online Edge Article Chemical Science material FeIII II 1/6Fe5/6(1,2,3-triazolate)2 1/6+ , wherein one sixth of all a consequence, VB electrons in Fe3+-incorporated Fe(1,2,3- Fe centers are replaced by Fe. Although this Fe3+ concen- 2+ 3+ triazolate)2 may be thermally activated into the mid-gap Fe- tration is much higher than experimentally observed in based states, promoting the formation of hole carriers in the Fe(1,2,3-triazolate)2, it simply articially increases the DOS VB. In addition, the spin density distribution in this hypothet- contributions from states arising from Fe3+ while simulta- ical material (Fig. 8b) shows that the spins, and equivalently the neously destabilizing the crystal. We were able to obtain a stable unpaired electrons, are partially delocalized among Fe centers. structure at this defect concentration and using a core level The Fe3+/2+ mixed valency should facilitate inter-iron charge alignment we were able to align the defective material to the hopping and improve charge mobility. We therefore attribute native Fe2+ framework. As shown in Fig. 8a, Fe3+ do not signif- the high electrical conductivity of Fe(1,2,3-triazolate)2 to the This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. icantly affect the energy of the native Fe(1,2,3-triazolate)2 bands. presence of mixed-valent Fe3+/2+. Instead, they give rise to mid-gap states attributed to the Fe d- Open Access Article. Published on 20 April 2017. Downloaded on 6/26/2024 7:08:57 AM. electron spin-down channels. These mid-gap states are found Discussion only 1.5 eV above EVBM. Such redox-accessible states are ex- pected to persist even at much lower Fe3+ concentration. As The unique role of Fe in promoting high electrical conductivity across four different families of MOFs that differ in both structure and organic connectivity is highlighted in Fig. 2. Although the particular reasons for this conserved role of Fe across different materials are likely convoluted, Fe stands out among the other metals considered here in several respects. First, among Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ the ionization energy of Fe2+ is the smallest at 30.652 eV (Table S5†).34 Second, the standard reduction potential (298 K) of the aqueous Fe3+/2+ couple, 0.771 V, is smaller than those of the aqueous Mn3+/2+, Co3+/2+, and Cu3+/2+ couples (Table S5†),35 whereas the trivalent states of the other metal ions are essen- tially inaccessible under similar experimental conditions.k Finally, owing to its large ionic radius and small effective nuclear charge, high-spin Fe2+ (as found in Fe2(DOBDC)(DMF)2, Fe2(DSBDC)(DMF)2, and Fe2Cl2(BTDD)(DMF)2) exhibits the smallest Coulombic attraction between its nucleus and its valence electrons (Table S5†). Together, these suggest that among the metal ions studied here, the valence electrons of high-spin Fe2+ have the highest energy. Because Fe orbitals dominate the VB of Fe2(DOBDC)(DMF)2, Fe2(DSBDC)(DMF)2, and Fe2Cl2(BTDD)(DMF)2, these high-energy electrons raise the EVBM and give rise to small Eg and Ea values. This subsequently leads to a higher probability of thermal activation at room temperature and higher charge density than available for the other metal analogs. The same arguments do not hold for low-spin Fe2+. Because low-spin Fe2+ and 1,2,3-triazolate do not contribute charge carriers, pure Fe(1,2,3-triazolate)2 should accordingly be elec- trically insulating. This is indeed predicted by DFT calculations, which show that pure Fe(1,2,3-triazolate)2 exhibits a larger Eg than its Mn2+, Co2+, and Cu2+ analogs (Fig. 7c). Instead, we attribute the observed high electrical conductivity of Fe(1,2,3- triazolate)2 to the presence of a small amount of Fe3+. The presence of Fe3+, and thus the formation of a mixed-valence Fe3+/2+ system was conrmed by EPR spectroscopy (Fig. 5). Furthermore, DFT calculations suggest that mid-gap states, which effectively lower Ea and increase electrical conductivity, Fig. 8 (a) Calculated energy bands and projected density of states (PDOS) become available upon forming Fe3+/2+ mixed valency in of native Fe(1,2,3-triazolate)2, and the hypothetical material Fe(1,2,3-triazolate)2. The presence of Fe3+ cannot be ruled out FeIII II 1/6Fe5/6(1,2,3-triazolate)2 1/6+. EVBM are shown on the top and band gaps for the high-spin Fe2+ materials. The inuence of Fe3+ would are shown in the middle of each sub-figure. (b) The spin density of the hypothetical material FeIII II 1/6+ mimic that observed for Fe(1,2,3-triazolate)2. Indeed, 1/6Fe5/6(1,2,3-triazolate)2 shows partially delocalized spin across the Fe centers (shown in yellow and red), with Fe2(DOBDC)(DMF)2, Fe2(DSBDC)(DMF)2, and some local Fe3+ character emphasized in green. Fe2Cl2(BTDD)(DMF)2 are signicantly more sensitive to O2 than This journal is © The Royal Society of Chemistry 2017 Chem. Sci., 2017, 8, 4450–4457 | 4455  View Article Online Chemical Science Edge Article Fe(1,2,3-triazolate)2, which makes the presence of trace Foundation, and the Research Corporation for Science amounts of Fe3+ in these materials likely. Because the oxidation Advancement (Cottrell Scholar Program) for non-tenured potential of the other metals are not as accessible, they are less faculty funds and the MISTI-Belgium fund for travel support. likely to be mixed valent under our experimental conditions. We thank Prof. S. J. Lippard for use of the Mössbauer spec- trometer, Dr M. A. Minier for help with collecting preliminary 57 Conclusions Fe Mössbauer spectra, and Dr D. Sheberla and A. W. Stubbs for helpful discussions. S. S. P. is partially supported by a NSF The foregoing results show a critical, conserved role of Fe in GRFP (Award No. 1122374). A. W. is supported by the Royal promoting high electrical conductivity across four different Society, the EPSRC (Grant EP/M009580/1) and the ERC (Grant This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. MOF families comprising twenty different materials and eight 277757). The computational work was facilitated by access to different metal ions. In each family, the Fe2+-based analog the UK National Supercomputer, ARCHER (EPSRC Grant EP/ Open Access Article. Published on 20 April 2017. Downloaded on 6/26/2024 7:08:57 AM. exhibits electrical conductivity and activation energy values that L000202) and access to the Extreme Science and Engineering are at least 5 orders of magnitude higher and 0.12–0.54 eV Discovery Environment (XSEDE), which is supported by smaller, respectively, than those of materials based on Mg2+, National Science Foundation Grant ACI-1053575. Mn2+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ ions. Both electronic structure and thermodynamic (i.e. redox accessibility) argu- ments explain the unique role of Fe within these eight metal ions. Similar arguments might provide hints for the design and Notes and references discovery of electrically conductive MOFs from other metal ions. ‡ In M2(DOBDC)(DMF)2, M2Cl2(BTDD)(DMF)2, and M(1,2,3-triazolate)2, the MOFs Most notably, Cr2+ is a promising candidate because it has are isostructural in each family, with the only difference among them being metal similar ionization energy and Coulombic attraction between its ions. Although Mn2(DSBDC)(DMF)2 and Fe2(DSBDC)(DMF)2 bear the same nucleus and valence electrons as Fe2+, as well as an accessible topology, the coordination environments of Mn and Fe differ. Whereas the former Cr3+/2+ redox couple (Table S5†). exhibits two crystallographically distinct Mn sites, the latter has only one crys- tallographically distinct Fe site (Fig. 1 and S2†). The connectivity in the (–Mn–S–)N More generally, our work demonstrates that mixed-valence chains is otherwise conserved, such that the subtle difference in the local coor- metal ions improve the electrical conductivity in MOFs. Mixed dination environment should not affect the electrical properties signicantly. valency is responsible for the high electrical conductivity in § For DFT calculations, the coordinating DMF molecules were removed for many inorganic solids,36 organic conductors,37,38 and coordina- M2(DOBDC) and M2(DSBDC). See detailed discussion in ESI.† tion polymers39 because it improves charge density and facili- { The spin states of Mn2+ ions in Mn2(DOBDC) and Mn(1,2,3,-triazolate)2 were tates charge delocalization. It is also applicable to MOFs, where reported to be S ¼ 5/2.14,45 Variable-temperature direct-current magnetic suscep- tibility measurements for Mn2(DSBDC), Co2(DOBDC), and Co(1,2,3-triazolate)2 both metal ions and organic ligands, if redox-active, can lead to also revealed high-spin ground states for the Mn2+ and Co2+ ions, respectively. See mixed-valent states.40,41 This has been shown already with two details in the ESI, Fig. S32–S34.† MOFs based on 1,2,4,5-tetrahydroxybenzene and its derivatives, k The inclusion of a Co3+ in a natively Co2+ framework is not expected to introduce where the ligands coexist in the semiquinone and quinone mid-gap states as the defect site would almost certainly be low spin. states, which gives rise to high electrical conductivity (103 to 101 S cm1).42,43 Therefore, redox-active metal ions and organic 1 V. Stavila, A. A. Talin and M. D. Allendorf, Chem. Soc. Rev., ligands are desirable when designing electrically conductive 2014, 43, 5994–6010. MOFs. 2 T. C. Narayan, T. Miyakai, S. Seki and M. Dincă, J. Am. Chem. Redox matching between metal ions and organic ligands is Soc., 2012, 134, 12932–12935. also critical to improve electrical conductivity in MOFs.44 This 3 S. S. Park, E. R. Hontz, L. Sun, C. H. Hendon, A. Walsh, requirement is not apparent in the materials studied here T. Van Voorhis and M. Dincă, J. Am. Chem. Soc., 2015, 137, because in all four families the ligands are small and neigh- 1774–1777. boring Fe centers have short interatomic distances (